Prebuilt systems

Install SteamOS. The system arrives with Windows or no operating system at all, and you write the official Valve image to a USB stick and reimage the drive. Budget an evening for it, most of which is waiting for downloads.

Often a little, because you also pay for assembly, testing and support, and you get less freedom in the parts. In exchange you skip the build and have a single warranty covering the whole machine.

Depends on the type. Mini PCs usually allow storage and memory changes only, because the graphics are part of the processor. Tower systems with a discrete card behave like a normal desktop and can take a new GPU later.

No. 1080p, 1440p and 4K describe what most current games do at reasonable settings, often with upscaling. A demanding release can sit well below the category it is listed under.

This site shows no fixed prices on purpose. Prebuilt pricing moves constantly and varies per configuration, so the price on the retailer page at the moment you order is the one that counts.